Semiconductor rolls

Abstract: A semiconductive roll is prepared by molding and curing a silicone rubber composition to a mandrel to form a semiconductive layer. The silicone rubber composition includes (A) 100 parts by weight of a diorganopolysiloxane, (B) 5 to 200 parts by weight of spherical silicone elastomer particles having a mean particle size of 0.1 to 100 µm, and (C) a conductive material, typically conductive carbon black, zinc oxide or titanium oxide. The roll has stable, minimal varying resistivity in the semiconductive region which does not depend on molding conditions.

Description: This specification relates to semiconductor rolls having a semiconductive rubber layer exhibiting electric resistance in the semiconductive range of 10 3 to 10 10 ohm, and to materials and methods for making them. A number of electroconductive rubbers are available as mixtures of electrically insulating rubbery material and electroconductive agents. For example, carbon black and other conductive agents are blended in rubbers to provide conductive rubbers with an electric resistivity of from 10 -1 to 10 2 Ω - cm, which are used in various applications. Silicone rubber is also widely used as electrically insulating rubber because of its heat resistance, low-temperature resistance and weatherability. It can also be used as conductive silicone rubber by adding conductive agents, as with other rubbery materials. The conductive agents which are added to silicone rubber for imparting electric conductivity are typically carbon black, graphite, various metal powders such as silver, nickel, and copper, various non - conductive particles and short fibers surface treated with silver or similar metals, carbon fibers, and metallic fibers. By mixing such conductive agents, the resistivity of silicone rubber can be reduced to the order of 10 10 to 10 -3 Ω-cm depending on the type and amount of conductive agent without detracting from the inherent properties of silicone rubber. When silicone rubber is blended with conductive carbon black such as Ketjen Black and acetylene black, however, the resulting compositions show great resistivity variations in the semiconductive region of 10 3 to 10 10 Ω-cm. it is difficult to provide stable electrical resistivity probably because the dispersion of carbon considerably varies with molding conditions. In the manufacture of business machine parts including those used in dry copying machines, for example, rubber rolls such as rolls for charging, transfer, developing, paper feed, fixing, pressing, electricity removal, cleaning, and oil application, the industry is now searching for materials having semiconductive characteristics. There is a need for a semiconductive roll which has stable resistivity with minimal variation in the semiconductor region. FR-A-2 637 604 describes the incorporation of spherical silicone rubber particles in highly-conductive silicone rubber compositions with the order of 10 Ω-cm. Proposed uses include antistatic rubber parts and fixing rollers in copying machines. The purpose of the spherical silicone particles is to increase the electrical conductivity so that less carbon black is needed and problems of workability and hardness associated with high carbon black loadings are reduced. JP-A-4/140772 describes making a conductive silicone rubber by compounding conductive silicone rubber grains into the silicone rubber stock. The rubber grains incorporate carbon black. The blend is moulded onto a roller shaft and vulcanised to make a developing roller. One aim is to achieve a uniform resistance of the silicone rubber EP 0558045 A1 (which is Article 54(3) EPC relation to the present case) described rolls having a semiconductive layer comprising organopolysiloxane, surfactant-treated inorganic filler, electrically conductive carbon block, a spherical silicone rubber powder and a curing agent. It identified the surfactant-treated inorganic filler as providing a narrow range of variation in volume resistivity and the silicone rubber powder as providing low tack and improved grindability. It would be desirable to be able to provide a novel and improved semiconductive rubber material and corresponding articles, e.g. rolls, having electrical resistivity with small variation in the semiconductive region. It would be desirable to have a stable (uniform) resistivity, with little or no dependence on molding conditions. A first aspect of the invention is a triboelectric charging roll, transfer roll or developing roll for developing apparatus, comprising a mandrel and a semiconductive layer enclosing the mandrel, said semiconductive layer being obtained by curing on the mandrel a curable silicone rubber composition comprising (A) 100 parts by weight of organopolysiloxane of the following average compositional formula (1): R n SiO (4-n)/2 wherein R is a substituted or unsubstituted monovalent hydrocarbon group and n is a positive number of 1.95 to 2.05, (B) 5 to 200 parts by weight of spherical silicone elastomer particles having a mean particle size or 0.1 to 100 µ m, and (C) electroconductive material being at least one selected from conductive carbon black, conductive zinc oxide and conductive titanium oxide whereby said semiconductive layer provides an electric resistance in the range from 10 3 to 10 10 Ω between its outer surface and the mandrel, the variation in said resistance measured at different points along the roll as described in the Examples being not more than two orders of magnitude, and within said range; and wherein the silicone rubber composition does not include surfactant-treated inorganic filler. A second aspect herein is the manufacture of a triboelectric changing roll, transfer roll or developing roll for developing apparatus by curing onto a mandrel a curable silicone rubber composition comprising (A) 100 parts by weight of organopolysiloxane of the following average compositional formula (1): R n SiO (4-n)/2 wherein R is a substituted or unsubstituted monovalent hydrocarbon group and n is a positive number of 1.95 to 2.05, and (C) electroconductive material being at least one selected from conductive carbon black, conductive zinc oxide and conductive titanium oxide to form a semiconductive layer having an electric resistance in the range 10 3 to 10 10 Ω between its outer surface and the mandrel and (B) 5 to 200 parts by weight of spherical silicone elastomer particles having a mean particle size of 0.1 to 100 µ m in the curable silicone rubber composition, to restrict variation of said resistance, measured at different points along the roll as in the Examples, to not more than two orders of magnitude within said semiconductive range of 10 3 to 10 10 Ω, and wherein the silicone rubber composition does not include surfactant-treated inorganic filler. In our tests, we achieved a resistance varying over only two orders of magnitude within the semiconductive region of 10 3 to 10 10 Ω. A semiconductive roll having such stable resistance is well suited for use in a developing section, especially as a triboelectric charging roll, transfer roll and developing roll in a developing apparatus. A general proposal herein is to use dispersed silicone elastomer particles, preferably spherical particles and preferably of mean particle size 0.1 to 100 µ m, to provide stable resistivity properties in an organopolysiloxane blended with a particle conductive material to give a resistivity in the semiconductor range. BRIEF DESCRIPTION OF THE DRAWING The only figure, FIG. 1 schematically illustrates how to measure the electric resistance of a semiconductive roll. DETAILED DESCRIPTION Component (A) of the silicone rubber composition is a diorganopolysiloxane, typically of average compositional formula (1): R n SiO (4-n)/2 In formula (1), letter n is a positive number of 1.95 to 2.05. R is a substituted or unsubstituted monovalent hydrocarbon group, preferably having 1 to 10 carbon atoms, especially 1 to 8 carbon atoms. Examples of the monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, and propyl groups; cycloalkyl groups such as cyclopentyl and cyclohexyl groups; alkenyl groups such as vinyl and allyl groups; cycloalkenyl groups; aryl groups such as phenyl and tolyl groups; and halogenated hydrocarbon groups and cyano-hydrocarbon groups wherein some or all of the hydrogen atoms are replaced by organic groups such as chlorine atoms, cyano groups or the like. Preferred organopolysiloxanes are those having a backbone of dimethylpolysiloxane units with or without phenyl, vinyl, 3,3,3-trifluoropropyl groups incorporated therein. The diorganopolysiloxane should preferably have alkenyl group e.g. in an amount of 0.02 to 5 mol% of R. The diorganopolysiloxane preferably has a degree of polymerization of at least 100 because below this level, the composition tends to be less moldable and the cured product has lower mechanical strength. Preferably, the degree of polymerization is in the range of 1,000 to 30,000, more preferably 3,000 to 10,000. Component (B) is particles of a silicone elastomer having a mean particle size of 0.1 to 100 µm, preferably 0.5 to 40 µm. They are preferably spherical or substantially spherical. Particles having a mean particle size of less than 0.1 µ m are difficult to prepare and less effective when added whereas particles having a mean particle size of more than 100 µ m can be detrimental to rubber's mechanical strength. The silicone elastomer particles are not limited in type, grade, manufacturing process and the like as long as they are of cured organopolysiloxane and have the above-defined particle size. Illustrative, nonlimiting examples of this component include silicone elastomer particles prepared by curing a curable organosiloxane composition in a spray dryer at 230 to 300°C as disclosed in JP - A 96122/1984; and silicone elastomer particles prepared by emulsifying a curable organosiloxane composition, for example, an addition reaction type organopolysiloxane composition comprising a vinyl - containing organopolysiloxane and an organohydrogenpolysiloxane in water with the aid of a surfactant so that emulsified particles may have a size of up to 20 µ m, adding a platinum series catalyst for addition reaction, and spray drying the emulsion, with the organopolysiloxane in the emulsified particles being cured before spray drying or until the end of spray drying as disclosed in JP - A 257939/1987. Preferred silicone elastomer particles are prepared by emulsifying a vinyl - containing organopolysiloxane and an organohydrogenpolysiloxane in water with the aid of a surfactant and effecting addition reaction in the presence of a platinum series catalyst for curing. The silicone elastomer particles may be surface-treated with silanes, siloxanes or the like previously or during their preparation. The amount of silicone elastomer particles used is 5 to 200 parts, preferably 10 to 150 parts by weight per 100 parts by weight of diorganopolysiloxane (A). Lower contents of elastomer particles are usually ineffective whereas a composition containing an excessive amount of elastomer particles may give a cured product with low mechanical strength. The electroconductive material (C) may be conductive carbon black, conductive zinc oxide, conductive titanium oxide, graphite, silver or nickel coated glass beads, or a mixture of two or more. Among them, conductive carbon black, conductive zinc oxide and conductive titanium oxide are preferred. Conductive carbon black may be selected from various types of carbon black commonly used in conductive rubber compositions. Examples include acetylene black, conducting furnace black (CF), super conducting furnace black (SCF), extra conducting furnace black (XCF), conducting channel black (CC), and furnace black and channel black heat treated at elevated temperatures of about 1,500°C. More particularly, the acetylene black includes Denka® Acetylene Black manufactured by Denki Kagaku K.K. and Shawinigan® Acetylene Black manufactured by Shawinigan Chemical Co., the conducting furnace black includes Continex® CF manufactured by Continental Carbon Co. and Vulcan® C manufactured by Cabot Corp., the super conducting furnace black includes Continex® SCF manufactured by Continental Carbon Co. and Vulcan® SC manufactured by Cabot Corp., the extra conducting furnace black includes Asahi® HS-500 manufactured by Asahi Carbon K.K. and Vulcan® XC-72 manufactured by Cabot Corp., and the conducting channel black includes Corax® L manufactured by Degussa Co. Also useful are modified furnace blacks commercially available as Ketjen® Black EC and Ketjen Black EC - 600JD from Ketjen Black International. Among these, acetylene black is advantageously used herein because it has a minimal impurity content and a well developed secondary structure. Ketjen Black EC and Ketjen Black EC - 600JD are also advantageous because high conductivity is available with a low loading due to their outstanding specific surface area. Conductive carbon black may be used singly or as admixture of two or more species, preferably in an amount of from 1 to 50 parts, more preferably from 5 to 20 parts by weight per 100 parts by weight of diorganopolysiloxane (A). Less than 1 parts of carbon black would sometimes fail to provide desired electric conduction whereas more than 50 parts of carbon black usually provides an electric resistance of less than 10 2 Q which falls outside the desired semiconductive region. Conductive zinc oxide is commercially available. For example, conductive zinc oxide commercially available from Honjo Chemical K.K. is useful. zinc oxide is preferably used in an amount of from 50 to 300 parts, more preferably from 80 to 250 parts by weight per 100 parts by weight of diorganopolysiloxane (A), thereby providing an electric resistivity of 10 5 to 10 10 Ω-cm. Less than 50 parts of zinc oxide would sometimes fail to provide desired electric conduction whereas more than 300 parts would adversely affect dynamic properties. Addition amounts of 100 to 250 parts by weight are especially preferred. Conductive titanium oxide is commercially available under the trade name ET-500W from Ishihara Industry K.K., for example. It preferably has a nominal composition of TiO 2 · SnO 2 doped with Sb. The amount of titanium oxide added is the same as that of zinc oxide. For curing of the silicone rubber composition, a curing agent is generally used. The curing agent may be selected from numerous well - known curing agents which are used in vulcanizing conventional conductive silicone rubber to induce vulcanization or curing through radical reaction, addition reaction or condensation reaction. Any curing mechanism is employed. Where the organopolysiloxane of formula (1) contains an alkenyl group, for example, useful are organic peroxides including alkyl peroxides such as di - t - butylperoxide and 2,5-dimethyl - 2,5 - di(t - butylperoxy)hexane and aralkyl peroxides such as dicumyl peroxide. The amount of the organic peroxide is preferably 0.1 to 5 parts by weight per 100 parts by weight of the organopolysiloxane of formula (1). A combination of a platinum series catalyst with an organohydrogenpolysiloxane having at least two hydrogen atoms each attached to a silicon atom in a molecule is a useful addition reaction curing agent. The organohydrogenpolysiloxane is preferably used in an amount such that the molar ratio of SiH group in the organohydrogenpolysiloxane to alkenyl group in the organopolysiloxane of formula (1) is in the range of 0.5 to 5. The platinum series catalyst is used in a catalytic amount. Where the organopolysiloxane of formula (1) contains a hydroxyl group, a polyfunctional alkoxysilane or siloxane and an organic metal acid salt may be used as a condensation curing agent. The amount of curing agent is approximately equal to that used with conventional conductive silicone rubber. If desired, the silicone rubber composition may further contain reinforcing silica fillers such as silica hydrogel (hydrous silicic acid) and silica aerogel (silicic anhydride fumed silica); fillers such as clay, calcium carbonate, diatomaceous earth, and titanium dioxide; dispersants such as low molecular weight siloxane esters and silanols, e.g., diphenylsilane diol; heat resistance enhancers such as iron oxide, cerium oxide and iron octylate; various carbon functional silanes for improving adhesion and workability; and platinum compounds for imparting flame retardancy. To form a semiconductive roll, a cured layer (silicone rubber layer) of the above-defined semiconductive silicone rubber composition is formed around a cylindrical mandrel or metal core. The material and size of the mandrel is suitably selected depending on the intended type of roll. Also the molding and curing methods of the silicone rubber composition are suitably selected. For example, molding can be done by any of pressure molding, transfer molding, extrusion molding, injection molding and calendering. The curing method depends on the type of curing agent used. The semiconductive cured layer or silicone rubber layer effectively provides a resistance of 1 × 10 3 to 1 × 10 10 Ω between its outer surface and the mandrel. In this range of resistance, the silicone rubber layer may provide an electric resistance varying within two orders of magnitude, preferably one order of magnitude. It is preferred as a semiconductive layer on triboelectric charging rolls, transfer rolls, developing rolls and paper feeding rolls in developing apparatus. In view of the teachings herein it is possible to make a roll covered with a silicone rubber layer having stable resistivity in the semiconductive region, with satisfactory independence of molding conditions, the resistivity providing low variation. The silicone rubber layer can be good in processability, vulcanizing properties, and rubbery elasticity. EXAMPLE Examples are given below by way of illustration and not by way of limitation. All parts are by weight. Examples 1-3 & Comparative Example 1 A silicone rubber composition was prepared by blending a methylvinylpolysiloxane consisting of 99.85 mol% of a dimethylsiloxane unit and 0.15 mol% of a methylvinylsiloxane unit and having an average degree of polymerization of about 8,000, treated silica ("R-972" of Nippon Aerosil K.K.), and spherical silicone elastomer particles ("KMP594": particle size 3-10 µ m, Shin-Etsu Chemical Co., Ltd.) in the amounts shown in Table 1, adding acetylene black, and kneading the mixture. Then an organic peroxide, 2,5-dimethyl - 2,5 - di(t - butylperoxy)hexane was added to and kneaded with the composition. The resulting compound was molded over and cured to a mandrel, obtaining a roll with an outer diameter of 20 mm. The molding conditions included a temperature of 165°C, a time of 15 minutes, and a pressure of 30 kgf/cm 2 . As shown in FIG. 1, the roll 1 included the elongated cylindrical mandrel 2 which over most of its surface was covered with the annular silicone rubber layer defining the roll surface. The roll (silicone rubber layer ) was measured for hardness and electric properties. The results are shown in Table 1. Hardness measurement is in accord with JIS K 6301. Electric resistance was measured, as shown in FIG. 1, by contacting a roll 1 with electrode strips 3 and measuring the resistance between each electrode strip 3 and the mandrel 2 by means of a resistance meter 4. Twenty electrode strips 3 of 7 mm wide were spaced 3 mm from each other. Measurement was thus done at 20 axially spaced - apart points to evaluate resistance variation. The measurement voltage was 100 volts and the meter 4 was a ultra-high resistance meter R8340 by Advanced Test Digital K.K. Example 4 - 5 & Comparative Example 2 Example 1 was repeated except that the acetylene black was replaced by conductive zinc oxide (Honjo Chemical K.K.) or white conductive titanium oxide ET-500W (Ishihara Industry K.K.) in the amount shown in Table 1. Hardness and resistance were measured, with the results shown in Table 1. Example Comparative Example 1 2 3 4 5 1 2 Methylvinylpolysiloxane 100 100 100 100 100 100 100 KMP-594 20 50 100 20 20 - 3 R-972 20 20 20 5 5 20 5 Acetylene black 13 13 13 13 Zinc oxide 200 200 Titanium oxide 200 Organic peroxide 1.6 1.6 1.6 1.6 1.6 1.6 1.6 Hardness 40 38 36 55 53 40 55 Resistance (Ω) 1 × 10 5 5 × 10 4 3 × 10 4 1 × 10 7 5 × 10 7 1 × 10 4 1 × 10 6 - - - - - - - 1 × 10 6 5 × 10 5 1 × 10 5 1 × 10 8 5 × 10 8 1 × 10 7 1 × 10 10 As is evident from Table 1, the silicone rubber compositions loaded with spherical silicone elastomer particles (Examples 1 - 5) provided stable resistance measurements in the semiconductive region as compared with the composition free of spherical silicone elastomer particles (Comparative Example 1) and the composition loaded with a smaller amount of spherical silicone elastomer particles (Comparative Example 2). In the new compositions, the resistance varied only over one order of magnitude.

Claims: A triboelectric charging roll, transfer roll or developing roll for developing apparatus, comprising a mandrel and a semiconductive layer enclosing the mandrel, said semiconductive layer being obtained by curing on the mandrel a curable silicone rubber composition comprising (A) 100 parts by weight of organopolysiloxane of the following average compositional formula (1): R n SiO (4-n)/2 wherein R is a substituted or unsubstituted monovalent hydrocarbon group and n is a positive number of 1.95 to 2.05, (B) 5 to 200 parts by weight of spherical silicone elastomer particles having a mean particle size or 0.1 to 100 µ m, and (C) electroconductive material being at least one selected from conductive carbon black, conductive zinc oxide and conductive titanium oxide whereby said semiconductive provides an electric resistance in the range from 10 3 to 10 10 Ω between its outer surface and the mandrel, the variation in said resistance measured at different points along the roll as described in the Examples being not more than two orders of magnitude, and within said range; and wherein the silicone rubber composition does not include surfactant-treated inorganic filer. A roll according to claim 1 wherein, as said electroconductive material, the curable silicone rubber composition contains from 1 to 50 parts by weight conductive carbon black per 100 parts by weight of the organopolysiloxane (A). A roll according to claim 1 wherein, as said electroconductive material, the curable silocone rubber composition contains from 50 to 30 parts by weight of conductive zinc oxide or conductive titanium oxide per 100 parts by weight of the organopolysiloxane (A). A roll according to any one of the preceding claims in which the silicone rubber composition is one in which the organopolysiloxane (A) includes alkenyl group and organic peroxide is used as curing agent. A roll according to any one of claims 1 to 3 in which the silicone rubber composition is an addition-cure composition in which the organopolysiloxane (A) includes alkenyl group and a curing agent comprises platinum series catalyst with organohydrogenpolysiloxane having per molecule at least two Si-bonded hydrogen atoms. A roll according to any one of the preceding claims in which the organopolysiloxane (A) is diorganopolysiloxane whose degree of polymerisation is in the range 1000 to 30000. A roll according to any one of the preceding claims in which the mean particle size of said spherical silicone elastomer particles is from 0.5 to 40 µ m. A roll according to any one of the preceding claims in which said resistance of the semiconductive layer varies over the layer, within said semiconductive range of 10 3 to 10 10 Ω, by not more than two orders of magnitude. The use, in the manufacture of a triboelectric changing roll, transfer roll or developing roll for developing apparatus by curing onto a mandrel a curable silicone rubber composition comprising (A) 100 parts by weight of organopolysiloxane of the following average compositional formula (1): R n SiO (4-n)/2 wherein R is a substituted or unsubstituted monovalent hydrocarbon group and n is a positive number of 1.95 to 2.05, and (C) electroconductive material being at least one selected from conductive carbon black, conductive zinc oxide and conductive titanium oxide to form a semiconductive layer having an electric resistance in the range 10 3 to 10 10 Ω between its outer surface and the mandrel, and (B) 5 to 200 parts by weight of spherical silicone elastomer particles having a mean particle size of 0.1 to 100 µ m in the curable silicone rubber composition, to restrict variation of said resistance, measured at different points along the roll as in the Examples, to not more than two orders of magnitude within said semiconductive range of 10 3 to 10 10 Ω and wherein the silicone rubber composition does not include surfactant-treated inorganic filler.

Inventors: NAKAMURA TSUTOMU, NUMATA KAZUO, NAKAMURA, TSUTOMU, NUMATA, KAZUO
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1994
Main classification: C08L83
IPC codes: G03G15, C08L83, F16C13, B65H5